1. Field of the Invention
This invention concerns a semiconductor device, in particular, a semiconductor device that is greatly improved in terms of the electrostatic breakdown voltage.
2. Description of the Related Art
The general consumer-use microwave device market, which began with the appearance of satellite broadcast receivers, has enlarged rapidly in scale with the worldwide spread of cellular phones and presently, the market for wireless broadband applications is about to emerge substantially. In these markets gallium arsenide (GaAs) devices, which are suited for microwave applications, and Si microwave devices, resulting from the making of conventional Si devices finer and arranging three-dimensional structures to achieve lower parasitic capacitance and lower parasitic resistance, are mainly used.
FIG. 13 is a circuit diagram of a compound semiconductor switching circuit device. The source electrodes (or drain electrodes) of first and second FET's, FET1 and FET2, are connected to a common input terminal IN, the gate electrodes of FET1 and FET2 are respectively connected to first and second control terminals Ctl-1 and Ctl-2 via resistors R1 and R2, and the drain electrodes (or source electrodes) of FET1 and FET2 are connected respectively to first and second output terminals OUT1 and OUT2. The control signals applied to the first and second control terminals Ctl-1 and Ctl-2 are complementary signals and the FET to which the H level signal is applied is made to turn ON and transmit the input signal applied to common input terminal IN to one of the output terminals. The resistors R1 and R2 are disposed for the purpose of preventing high-frequency signals from leaking via the gate electrodes to the DC potential of the control terminals Ctl-1 and Ctl-2 which are AC grounded.
FIG. 14 shows an example of a compound semiconductor chip designed by integrating this compound semiconductor switching circuit.
FET1 and FET2, which perform switching, are disposed at central parts of a GaAs substrate, and the resistors R1 and R2 are connected to the respective gate electrodes of the FET's. Also, pads, respectively corresponding to the common input terminal IN, the output terminals OUT1 and OUT2, and the control terminals Ctl-1 and Ctl-2, are disposed at peripheral parts of the substrate. A gate metal layer (Ti/Pt/Au) 20, which is formed at the same time as the forming of the gate electrodes of the respective FET's, is a second-layer wiring and indicated by dotted lines, and a pad metal layer (Ti/Pt/Au) 30, which connects the respective elements and form the pads, is a third-layer wiring, indicated by solid lines. An ohmic metal layer (AuGe/Ni/Au) of a first layer wiring, which is in ohmic contact with substrate, forms the source electrodes and drain electrodes of the respective FET's and take-out electrodes at the ends of the respective resistors, and this layer is not illustrated in FIG. 14 as it overlaps with the pad metal layer.
FET1, shown in FIG. 14, is formed in a rectangular operating region 12 that is surrounded by alternate long and short dash lines. The three third-layer pad metal layer 30 parts, which take on the form of comb teeth that extend from the lower side, are a source electrode 13 (or drain electrode) connected to output terminal OUT1, and below this is disposed a source electrode 14 (or drain electrode), formed by a first-layer ohmic metal layer 10. Also, the three third-layer pad metal layer 30 parts, which take on the form of comb teeth that extend from the upper side, are a drain electrode 15 (or source electrode), connected to common input terminal IN, and below this is disposed a drain electrode 14 (or source electrode), formed by the first-layer ohmic metal layer 10. These electrodes are disposed in the form of engaged comb teeth and a gate electrode 17, formed on an operating region 12 from the second-layer gate metal layer 20, is disposed in between in the form of five comb teeth. A central drain electrode 15 (or source electrode) that extends from the upper side is used in common by FET1 and FET2 to further contribute to size reduction. Here, that the gate width is 600 μm means that the total gate width of the comb-teeth-like gate electrode 17 of each FET is 600 μm.
As described above, with a conventional switching circuit device, no measures are taken in particular for protection against electrostatic breakdown.
FIG. 15 shows the results of measuring the electrostatic breakdown voltage of the switching circuit device shown in FIG. 14. Here, the electrostatic breakdown voltage is measured under the following conditions. That is, after applying a test voltage to the terminals of a test capacitor of 220 pF and thereby charging electricity in the test capacitor, the wiring for voltage application is cut off. Thereafter, the charged electricity in the test capacitor is discharged between the terminals of a tested element (FET) without resistance factor nor inductance factor inserted and whether or not the FET breaks down is measured. If the FET does not break down, the test is repeatedly carried out with the first application voltage being increased by 10V at a time, and the application voltage at which the FET breaks down is measured as the electrostatic breakdown voltage.
As is clear from this figure, since measures for improving the electrostatic breakdown voltage are not taken in the conventional art, the electrostatic breakdown voltage is the lowest and only 140 V between common input terminal IN and control terminal Ctl-1 and between common input terminal IN and control terminal Ctl-2.
Also, there is variation of the electrostatic breakdown voltage depending between which terminals the electrostatic breakdown voltage is measured. Though the specific mechanism that determines this electrostatic breakdown voltage is unclear, with a switching circuit device, the value of the minimum electrostatic breakdown voltage between two terminals is generally of the level of 100 V or less as mentioned above, and the finest care was required for handling. That is, the electrostatic breakdown voltage value for the terminals between which the electrostatic breakdown voltage is the minimum governs the electrostatic breakdown voltage of the element as a whole, and thus improvement of the electrostatic breakdown voltage between these terminals is the subject.
Besides, unlike other devices for audio, video and power supply applications, microwave communication devices are low in the internal Schottky junction or PN junction capacitance and these junctions are weak against static electricity.
Generally in order to protect a device against static electricity, a method, in which an electrostatic breakdown protecting diode is connected in parallel between the terminals of a PN junction or Schottky junction that is damaged readily by electrostatic discharge, in a device, may be considered. However, this method could not be applied to a microwave device since increased parasitic capacitance due to connection of a protecting diode causes degradation of the high-frequency characteristics.